Three-dimensional imaging using MRI equipment is inherently slow. Both the acquisition and reconstruction processes consume more time than equivalent two dimensional multi-slice acquisition and reconstruction. In fact because of the inherent slowness of three-dimensional imaging, such imaging is often avoided. However, since three-dimensional imaging can play a key role in such fields as angiography and volumetry it is worthwhile to speed up the three-dimensional imaging process; i.e., to make the 3-D imaging process including the acquisition and reconstruction more efficient.
In addition, three-dimensional techniques offer other benefits. For example, the three-dimensional techniques enable acquisition of very thin contiguous slices (down to 1 mm) with minimal inter-slice crosstalk and provide potentially high signal-to-noise ratios (SNR). SNR, as is well known, increases as the square root of the number of phase encodings (perpendicular to the image plane); therefore, a 32 slice three-dimensional scan has a little over five times the signal-to-noise ratio of its two-dimensional counterpart, assuming all other parameters to be equal.
The potential usefulness of three-dimensional imaging has inspired those skilled in the art to adapt many of the fast scan procedures to three-dimensional imaging. For example, in an article entitled "Introduction to Fast Scan Magnetic Resonance" by Felix W. Wehrli, Ph.D., The General Electric Company's Gradient recall fast scan technique is described as being applied to three-dimensional volumetric data acquisition.
In addition to speeding up the acquisition, those skilled in the art are searching for ways of speeding up reconstruction or improving the reconstruction efficiency in three-dimensional imaging procedures. Accordingly, it is an object of the present invention to maximize the efficiency of the three-dimensional reconstruction techniques.
Efficiency of three-dimensional imaging is measured by:
1) the time to the appearance of the first image measured from the completion of the acquisition, and PA1 2) the rate of appearance of images. PA1 (a) subjecting the patient to a large static magnetic field to align "spins" in the patient with the large static magnetic field, PA1 (b) applying an RF pulse to "tip" spins from their aligment with the large static magnetic field, PA1 (c) applying an encoding gradient pulse in the Y direction, PA1 (d) applying an encoding gradient pulse in the Z direction, PA1 (e) applying a frequency encoding gradient pulse in the X direction to generate FID signals in the patient, PA1 (f) receiving, sampling and recording said FID signals from the patient, PA1 (g) repeating steps (b)-(f) Nz times with the same Y encoding gradient pulse in step (c) and Nz different Z encoding gradient pulses in sted (d), PA1 (h) Fourier transforming the FID signals in the Y direction Nx times, PA1 (i) recording the results in Nz separate two-dimensional matrices, each matrix corresponding to an image in the XY plane, PA1 (j) repeating steps (b)-(i) Ny times with different Y encoding gradient pulses in step (c) and each time recording additional results into the Nz separate matrices, PA1 (k) selecting an Nz matrix for processing, PA1 (l) Fourier transforming the data in the selected matrix in both dimensions to obtain image data, PA1 (m) using the image data to provide an image, and PA1 (n) repeating steps (k)-(m) for all Nz matrices until all Nz XY images are reconstructed.